have you looked at the MYOG: 5 yards to SUL article series? The backpack pattern (part 4 i think) is pretty easy. The pack is a little bigger then you are looking for at ~27L, but since you are making it i think that it would be an easy mod to just make it a little smaller. Adding a hip belt, a bladder sleeve and using tougher material, and i think it would work just fine for biking with.

A number of the members make their own gear and a couple have made backpacks. The most popular commercial model used by that community seems to be the Osprey Talon 22. It has an excellent harness system which you might want to copy.

Get a cheap kids backpack at the thrift store and a drybag style stuff sack from your local outdoors store. Cut the pack body off, leaving just a backpanel and shoulder straps. Sew your drybag onto that. Saves hours of sewing. Or check out CiloGear's 20L worksack. I use it for mountain biking and climbing.

Have seen the CiloGear 20L. Look pretty nice. I prefer a padded backpanel for mountain biking. I though about the dry bag + kid backpack.

I took several ideas and am sewing one right now. I will post picture when done. It will not be as light as some BUT it will be mine and adapted for my use. Much work than i thought. In fact, the padded backpanel tooked more time than i was thinking. Everything else is not too tough.

Here my new backpack. You can see more at http://guybouchard.wordpress.com/2010/03/10/projet-sac-a-dos-20l-backpack-project/ . Weight? Can't tell but pretty good. Don't forget, i will use it for day skiing so i need to operate it with glove and mitts. Can't wait to test it!